Andesite ()
Andesite is the extrusive equivalent of plutonic diorite. Characteristic of subduction zones, andesite represents the dominant rock type in . The average composition of the continental crust is andesitic.
The name andesite is derived from the Andes mountain range, where this rock type is found in abundance. It was first applied by Christian Leopold von Buch in 1826.
Description
Andesite is an
aphanitic (fine-grained) to
porphyritic (coarse-grained) igneous rock that is intermediate in its content of
silica and low in
alkali metals. It has less than 20% quartz and 10%
feldspathoid by volume, with at least 65% of the
feldspar in the rock consisting of
plagioclase. This places andesite in the
basalt/andesite field of the
QAPF diagram. Andesite is further distinguished from basalt by its silica content of over 52%.
However, it is often not possible to determine the mineral composition of volcanic rocks, due to their very fine grain size, and andesite is then defined chemically as volcanic rock with a content of 57% to 63% silica and not more than about 6% alkali metal oxides. This places the andesite in the O2 field of the TAS classification. Basaltic andesite, with a content of 52% to 57% silica, is represented by the O1 field of the TAS classification but is not a distinct rock type in the QAPF classification.
Andesite is usually light to dark grey in colour, due to its content of hornblende or pyroxene minerals.[Philpotts and Ague 2009, p. 139]
The plagioclase in andesite varies widely in sodium content, from anorthite to oligoclase, but is typically andesine, in which anorthite makes up about 40 mol% of the plagioclase. The pyroxene minerals that may be present include augite, pigeonite, or orthopyroxene. Magnetite, zircon, apatite, ilmenite, biotite, and garnet are common accessory minerals.
Andesite is usually porphyritic, containing larger crystals () of plagioclase formed prior to the extrusion that brought the magma to the surface, embedded in a finer-grained matrix. Phenocrysts of pyroxene or hornblende are also common.[Blatt and Tracy 1996, p.57] These minerals have the highest melting temperatures of the typical that can crystallize from the melt
Andesitic volcanism
Andesite lava typically has a viscosity of 3.5 million
centipoise (3500 Pa⋅s) at . This is slightly greater than the viscosity of smooth
peanut butter. As a result, andesitic volcanism is often explosive, forming
and
. Andesite vents tend to build up composite volcanoes rather than the
characteristic of basalt, with its much lower viscosity resulting from its lower silica content and higher eruption temperature.
Block lava flows are typical of andesitic lavas from composite volcanoes. They behave in a similar manner to ʻaʻā flows but their more viscous nature causes the surface to be covered in smooth-sided angular fragments (blocks) of solidified lava instead of clinkers. As with ʻaʻā flows, the molten interior of the flow, which is kept insulated by the solidified blocky surface, advances over the rubble that falls off the flow front. They also move much more slowly downhill and are thicker in depth than ʻaʻā flows.
Generation of melts in island arcs
Though andesite is common in other tectonic settings, it is particularly characteristic of convergent plate margins. Even before the Plate Tectonics Revolution, geologists had defined an
andesite line in the western Pacific that separated
basalt of the central Pacific from andesite further west. This coincides with the
at the western boundary of the
Pacific Plate.
Magmatism in
island arc regions comes from the interplay of the
Subduction and the
mantle wedge, the wedge-shaped region between the subducting and overriding plates. The presence of convergent margins dominated by andesite is so characteristic of the Earth's unique
that the Earth has been described as an "andesite planet".
During subduction, the subducted oceanic crust is subjected to increasing pressure and temperature, leading to metamorphism. Hydrate minerals such as amphibole, , or Chlorite group (which are present in the oceanic lithosphere) dehydrate as they change to more stable, anhydrous forms, releasing water and soluble elements into the overlying wedge of mantle. Fluxing water into the wedge lowers the solidus of the mantle material and causes partial melting. (1995). 086542361X, Blackwell Scientific. 086542361X
Basalt thus formed can contribute to the formation of andesite through fractional crystallization, partial melting of crust, or magma mixing, all of which are discussed next.
Genesis
Intermediate volcanic rocks are created via several processes:
-
Fractional crystallization of a mafic parent magma.
-
Partial melting of crustal material.
-
Magma mixing between felsic rhyolite and mafic magmas in a magma reservoir
-
Partial melting of metasomatized mantle
Fractional crystallization
To achieve andesitic composition via fractional crystallization, a basaltic magma must crystallize specific minerals that are then removed from the melt. This removal can take place in a variety of ways, but most commonly this occurs by crystal settling. The first minerals to crystallize and be removed from a basaltic parent are
and
. These mafic minerals settle out of the magma, forming mafic cumulates.
There is geophysical evidence from several arcs that large layers of mafic cumulates lie at the base of the crust.
Once these mafic minerals have been removed, the melt no longer has a basaltic composition. The silica content of the residual melt is enriched relative to the starting composition. The
iron and
magnesium contents are depleted. As this process continues, the melt becomes more and more evolved eventually becoming andesitic. Without continued addition of mafic material, however, the melt will eventually reach a
rhyolite composition. This produces the characteristic basalt-andesite-rhyolite association of island arcs, with andesite the most distinctive rock type.
Partial melting of the crust
Partially molten basalt in the mantle wedge moves upwards until it reaches the base of the overriding crust. Once there, the basaltic melt can either
Underplating the crust, creating a layer of molten material at its base, or it can move into the overriding plate in the form of dykes. If it underplates the crust, the basalt can (in theory) cause partial melting of the lower crust due to the transfer of heat and volatiles. Models of heat transfer, however, show that arc basalts emplaced at temperatures 1100–1240 °C cannot provide enough heat to melt lower crustal
amphibolite.
Basalt can, however, melt
Pelite upper crustal material.
Magma mixing
In continental arcs, such as the
Andes, magma often pools in the shallow crust creating magma chambers. Magmas in these reservoirs become evolved in composition (dacitic to rhyolitic) through both the process of fractional crystallization and partial melting of the surrounding country rock.
Over time as crystallization continues and the system loses heat, these reservoirs cool. In order to remain active, magma chambers must have continued recharge of hot basaltic melt into the system. When this basaltic material mixes with the evolved rhyolitic magma, the composition is returned to andesite, its intermediate phase.
Evidence of magma mixing is provided by the presence of phenocrysts in some andesites that are not in chemical equilibrium with the melt in which they are found.
Partial melting of metasomatized mantle
High-magnesium andesites (
) in island arcs may be primitive andesites, generated from metasomatized mantle.
[Kelemen, P.B., Hanghøj, K., and Greene, A.R. "One View of the Geochemistry of Subduction-Related Magmatic Arcs, with an Emphasis on Primitive Andesite and Lower Crust." In Treatise on Geochemistry, Volume 3. Editor: Roberta L. Rudnick. Executive Editors: Heinrich D. Holland and Karl K. Turekian. pp. 659. . Elsevier, 2003., p.593-659]
Experimental evidence shows that depleted mantle rock exposed to alkali fluids such as might be given off by a subducting slab generates magma resembling high-magnesium andesites.
Notable andesite structures
Notable
stonemasonry structures built with andesite include:
-
Borobudur in Java, Indonesia
-
Sacsayhuamán citadel in Peru
-
Gate of the Sun in Bolivia
-
Templo Mayor ruins and other historic buildings in Mexico City, built from andesite and Tezontle basaltic andesite
Extraterrestrial samples
In 2009, researchers revealed that andesite was found in two meteorites (numbered GRA 06128 and GRA 06129) that were discovered in the
Graves Nunataks icefield during the US
ANSMET 2006/2007 field season. This possibly points to a new mechanism to generate andesite crust.
Along with basalts, andesites are a component of the Martian crust.
See also
External links
